1. Field of the Invention
The present invention relates to a flowmeter suited for measuring a small flow rate of fluid, and more particularly to a flowmeter suited for measuring a small flow rate of fluid in a semiconductor manufacturing apparatus for manufacturing, for example, a semiconductor device having a fine interconnect structure.
2. Description of the Related Art
For measuring the flow rate of a fluid, such as a gas or liquid, a flowmeter can be used which has a heating element disposed in a casing and measures the flow rate of a fluid by measuring the temperature of the heating element whose temperature changes with the flow rate of the fluid flowing in the casing.
A flowmeter is known which includes a plurality of light emitting diodes (LEDs) disposed on one side of a casing formed of an optically-transparent material, a plurality of light-receiving photodiodes disposed on the opposite side of the casing and facing the LEDs, and a float formed of a light-shielding material disposed in the casing. The flowmeter detects the position of the float by detecting the position of a light-receiving photodiode which has come to receive no light from an LED due to blocking of light by the float (see, for example, Japanese Patent laid-Open Publication No. H2-388162).
Another flowmeter is known which includes a float formed of a light-shielding material disposed in a casing formed of an optically-transparent material, and a plurality of imaging sensor units, each comprised of an optical lens and an image sensor, disposed along the casing to recognize an effective imaging range for the casing by a combination of the imaging ranges of the imaging sensor units. This flowmeter detects the position of the float based on electrical signal outputs from the imaging sensor units (see, for example, Japanese Patent Laid-Open Publication No. 2001-221666).
A similar flowmeter is known which uses lenses and CCD line sensors to project a one-dimensional image of a float on the line sensors and detects the position of the float from the position of the image (see, for example, Japanese Patent Laid-Open publication No. 2001-221666).
Further, another flowmeter is known which includes a float provided with a permanent magnet, a casing, and a plurality of magnetic sensors disposed outside the casing, and which detects the position of the float by detecting the magnetism of the permanent magnet of the float with one of the plurality of magnetic sensors (see, for example, Japanese Patent laid-open Publication No. H11-190644). In the conventional flowmeter using a heating element manufactured, for example, with a micromachine, the heating element is exposed to a fluid. When handling a corrosive fluid, it is therefore necessary to cover and protect the heating element with a protective material. In this case, because of the heat conductivity of the protective film, the measurement precision cannot be made high in measurement of a small flow rate of fluid. In addition, the response speed undesirably becomes slow, leading to difficult measurement of a small flow rate of fluid.
In the case of the conventional flowmeter having optical or magnetic sensors provided outside a casing for detecting the position of a float, the number of parts must be increased in order to enhance the measurement precision. This leads to difficult assembly, a larger-sized construction, and a higher cost. Further, a flowmeter having optical sensors needs the use of an optically-transparent casing, and therefore materials for the casing are restricted. In addition, such a flowmeter deals with some types of fluids with difficulty.